1. Field of the Invention
The present invention relates to electromagnetic direction finding and, more particularly, to determining the azimuth and elevation angle of an emitter using a rotating interferometer wheel.
2. Description of the Related Art
In known interferometer direction finding systems, a phase difference between signals received at two antennas is used to determine the angle of arrival of a signal at the interferometer. One such system is disclosed in U.S. Pat. No. 4,845,502 to Carr et al. In such a system, however, the measured phase difference becomes ambiguous as the distance between the two antennas increases beyond .lambda./2. This problem is schematically illustrated in the FIG. 1 diagram. In FIG. 1, antennas 10 and 15 receive a signal at a given angle .phi.. As the distance S increases beyond .lambda./2, the measured phase difference .psi..sub.j becomes ambiguous and is expressed by the following: EQU .psi..sub.j =(2.pi.S/.lambda.) cos .phi.modulo 2.lambda. (1)
where .phi. represents a cone angle corresponding to the true unambiguous phase difference and .lambda. is the wavelength. Of course, it is possible to physically position the antennas 10 and 15 so that they are spaced not more than .lambda./2 apart, if the antennas are sufficiently physically narrow. However, a shorter spacing yields less direction finding accuracy than a longer spacing. In typical systems a large bandwidth is normally desirable such as in the range of 2-18 GHz. To obtain such a bandwidth, a common spiral antenna has a diameter of approximately 2 inches. However, the wavelength at 18 GHz is about 0.656 inches. Thus, an interferometer with a bandwidth in the range of, for example, 2-18 GHz would have the antennas spaced more than three wavelengths apart at 18 GHz, if the antennas were placed as closely together as physically possible. Consequently, the measured phase .psi..sub.j would not unambiguously correspond to a cone angle.